A planar lightwave circuit (PLC) is the optical equivalent of an electronic chip, manipulating and processing light signals rather than electronic signals. In most cases, a PLC is formed in a relatively thin layer of glass, polymer or semiconductor deposited on a semiconductor substrate. The lightwave circuit itself is composed of one or more optical devices interconnected by optical waveguides, the waveguides functioning to guide the light from one optical device to another and therefore considered as the optical equivalent of the metal interconnections in an electronic chip. The optical devices may comprise either passive optical devices, or active electro-optic devices, performing functions including, for example, reflection, focusing, collimating, beam splitting, wavelength multiplexing/demultiplexing, switching modulation and detection, and the like.
The common platforms used for the integration of lightwave devices are based upon using InP, silica-on-silicon, polymer and silicon oxynitride. The ability to integrate electronic components with these optical devices has always been extremely limited, since the majority electronic integrated circuit technology is based upon a silicon platform, rather than any of the above-mentioned platforms used for lightwave devices. A true integration of optics and electronics could be achieved, therefore, through the formation of optical devices in a silicon platform. A candidate system for such an integration is the silicon-on-insulator (SOI) structure, which allows for the guiding of light in the same surface single crystal silicon layer (hereinafter referred to as the “SOI layer”) that is used to form electronic components.
As of now, the common planar optical devices formed in an SOI structure use a relatively thick (>3-4 μm) SOI layer, so as to allow for relative ease of input/output coupling of a lightwave signal to the SOI layer through methods such as edge illumination. However, the need for edge-illumination coupling requires access to the edge of the chip, as well as the formation of an edge with a high surface quality. Further, the fabrication of high definition structures is considered to be rather difficult in a thick SOI layer (for example, forming “smooth” vertical walls for waveguides, rings, mirrors, etc). The thickness of the silicon also prevents the use of conventional CMOS fabrication processes to simultaneously form both the electronic and optical components. Moreover, the thickness of the SOI layer also limits the speed of the electronic devices.
Once the thickness of the SOI layer drops below one micron (which would be preferable to address the above-described problems), a significant challenge remains in terms of coupling a sufficient amount of a lightwave signal into and out of such a relatively thin layer. Several methods being explored for use in coupling light into a thin SOI layer include waveguide gratings, inverse nano-tapers and three-dimensional horn tapers. However, the coupled light propagates in the SOI layer with only vertical confinement (slab waveguide). The propagation of light in the lateral direction is similar to that in free space, with the refractive index of the medium equal that of silicon. In order to make a practical use of the coupled light, the need remains to effectively manipulate the light in the sub-micron SOI layer. More particularly, the need exists to perform various optical functions, such as turning, focusing, modulating, attenuating, switching and selectively dispersing the light coupled in the silicon layer. For a true integration of optics and electronics, all of these optical functions need to be achieved without exiting the planar waveguide structure.